[LLVMdev] Status of pre-legalize vector changes

[Chris Lattner]

On Mon, 11 Jun 2007, Dan Gohman wrote:
> Here's a quick update on where I'm at with the pre-legalize vector changes
> I'm working on. I hope to have an updated patch ready to a few days, assuming
> I don't get too busy with other projects going on. Here are some of the issues
> I've come across so far.

Great!  I'm going to reorder your email a bit to suit my evil purposes:

> Also, CopyToReg and CopyFromReg lowering/legalization rely on being able to
> create vector-of-vector types. For example, an <8 x double> vector is bitcasted
> to (effectively) <4 x <2 x double>> for x86, allowing EXTRACT_ELEMENT to pull
> out whole legal vectors at a time instead of just individual elements, for
> example.

Hrm, now that you mention it, I do remember that.  I'd file that into a 
category of "evil hacks", not something that we really want to support. 
To me, I think it would be much better to fix this directly, rather than 
complicate your implementation.

In particular, you should be able to introduce a new [V]CONCAT_VECTOR 
node, which takes two input vectors and yields an output vector that has 
length equal to the sum of the input vectors.  Likewise, instead of the 
extract_element hack, we should have an EXTRACT_SUBVECTOR node of some 
sort.

Since no target support these nodes, they would all be expanded by 
legalize.

Note that this change is logically independent of the rest of your change, 
so you could do this on mainline, as a first step to getting the bigger 
change in.

> While my original patch made the extended ValueType table explicitly
> hold vector element types and vector lengths, I had converted it to hold
> Type*, and ran into problems because VectorType doesn't permit the element
> types to be vectors. It seems unfortunate to give up using Type* just because
> of these few cases, but at the moment I don't have any simpler alternatives.

Vectors of vectors really is a hack.  Type*'s seems like a good way to go.

> Putting the table for extended ValueTypes in SelectionDAG seems quite involved.
> There are a lot of places that use the MVT-namespace functions, and making
> sure they all have access to a SelectionDAG object in order to use them is a
> lot of changes. It's tempting to make the table be a ManagedStatic, in
> VMCore/ValueTypes.cpp, not unlike the ManagedStatic objects in VMCore/Type.cpp,
> for example.

The problem with this approach is that it makes it much harder to codegen 
two different functions in parallel on different threads.  This isn't 
something we do today, but is something we want to do eventually. 
Requiring synchronization in such a critical datastructure would be 
badness...

That said, I don't know just how horrible the impact is on the code. 
Maybe there is another way?  Maybe the value table should be a first class 
datastructure that SelectionDAG contains.  This would let you do things 
like:

SD.getValueTable().getNumVectorElements(VT)

but it would also allow clients to grab a reference to the valuetable and 
call into it directly if more convenient.  Would this help at all?

One nice thing about this is that the ValueTable datastructure would be 
very simple, and moving all the methods to operate on a value table would 
be straight-forward.  If ValueTable was a simple class, then tblgen could 
instantiate it (f.e.) and not populate it with any of the funny MVT's.

-Chris

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